Low pressure radio frequency (RF) generated plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor device fabrication processes including surface treatments, depositions, and etching processes. For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created within the plasma impact the surface of the target to dislodge, i.e., "sputter" material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate being deposited at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer. To prevent such cavities, the sputtered material can be "collimated" into substantially vertical paths between the target and the substrate by negatively charging the substrate and positioning appropriate vertically oriented collimating electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However, material sputtered by a low density plasma often has an ionization degree of less than 1% which is usually insufficient to avoid the formation of an excessive number of cavities. Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation degree of unwanted cavities in the deposition layer. As used herein, the term "dense plasma" is intended to refer to one that has a high electron and ion density.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil surrounding the plasma induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the first windings of a transformer. The plasma acts as a single turn second winding of a transformer.
This known apparatus for forming a plasma discharge suffers from various disadvantages. In particular, power absorption in the plasma is usually localized to a region just a few skindepths (typically a few cm) from the outside edge of the plasma such that the interior of the plasma generally absorbs substantially less power than the outer edge of the plasma. As a consequence, plasma excitation is nonuniform which may result in nonuniform etching or deposition.
It is recognized that in a conventional Inductively Coupled Plasma (ICP) generator using a helical coil, such as that shown in U.S. Pat. No. 4,362,632, the electromagnetic energy radiating from each turn of the coil antenna is in phase. Also, fields are coupled into the plasma in a substantially pure inductive mode. The density is usually limited to .ltoreq.10.sup.11 14 10.sup.12 cm.sup.-3.
In contrast, a plasma excited using wave heating (helicon and ECR discharges) can be excited to densities as high as several 10.sup.13 cm.sup.-3 and thus wave heating is preferred where a more dense plasma is required. Such helicon waves are absorbed much more uniformly throughout the discharge. Helicon waves can be excited in a weakly magnetized (typically B&lt;500 gauss) plasma by means of a properly constructed antenna. In its simplest form, the helicon m =0 mode can be excited by two coil windings where the currents in each winding are in opposite directions.
An example of a known apparatus for utilizing helicon waves to generate plasmas of high density is shown in U.S. Pat. No. 4,990,229 to Campbell et al. U.S. Pat. No. 4,990,229 teaches that the efficient generation of plasmas depends strongly on the antenna configuration used. In other words, to maximize helicon wave coupling, a very specific and sometimes complex and large antenna configuration is often necessary. FIG. 2 of U.S. Pat. No. 4,990,229 depicts a two loop antenna used to excite the m=0 helicon mode. It is believed that the distance between the two loops is adjusted to match the m=0 helicon dispersion relation, i.e., ##EQU1##
where e is the charge of an electron;
.mu..sub.o is the permittivity;
.omega..sub.c is the electron cyclotron frequency (eB.sub.o m.sub.e); PA1 .omega..sub.p is the plasma frequency ##EQU2## PA1 m.sub.e is the mass of an electron; PA1 k.sub.z =2.pi./.lambda..sub.z is the wavenumber in axial direction; PA1 a is the radius of the plasma; PA1 L is the distance between the loops; PA1 n.sub.o is the plasma density, PA1 .omega.=2.pi.f is the excitation angular frequency; PA1 B.sub.o is the axial magnetic field; and PA1 .epsilon..sub.o is the permittivity of vacuum.
It is believed that for particular conditions (.omega., n.sub.o, B.sub.o, a) the distance L between the loops of the antenna for efficient coupling of the helicon wave is fixed by this dispersion relation. In the approximation of k.sub.z &lt;&lt;3.83/a, equation (1) can be rewritten as: ##EQU3##
for typical conditions (B.sub.o /n.sub.o =5.times.10.sup.-10, f=13.6 MHz, a=15 cm) one obtains .lambda.z=75 cm. This means that the distance between the two loops is restricted to about 40 cm for efficient coupling of the m=0 helicon mode. This would lead to a reactor aspect ratio of about unity. For large size substrates like TFT glass or silicon wafers, this would lead to an inconveniently large reactor volume. Also, the target to wafer spacing would often result in being about the same as the chamber diameter which would make it more difficult to efficiently produce uniform films on a wafer.
Examples of other geometrically complex antenna structures required to establish the electromagnetic fields necessary to launch the helicon wave are illustrated in FIGS. 3 and 5 of U.S. Pat. No. 4,990,229. Such complex and often large geometries are believed necessary in such prior art systems because most other variables affecting helicon wavelength and coupling efficiency are fixed by other constraints. Antenna geometry is one of the few variables which may be somewhat more easily modified in order to establish an appropriate electromagnetic field. For realizing both efficient coupling of the wave energy to the electron gas, and flexible geometry, it would be desirable to have independent control over k.sub.z or .lambda..sub.z.
U.S. Pat. No. 5,146,137 describes various devices for the generation of a plasma using helicon waves. These waves are generated in one device using four or more plate-like electrodes surrounding a quartz chamber containing the plasma. The electrodes are coupled to a voltage source through phase shifters to produce high frequency capacitively-coupled voltages having a phase rotation of 900.degree.. In an alternative device, four or more toroid-shaped coils are coupled to voltage sources to inductively couple electromagnetic energy into the chamber. The electrodes and coils of this reference also appear to be relatively complex.
In a number of deposition chambers such as a physical vapor deposition chamber, the chamber walls are often formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, it is often necessary to place the antenna coils or electrodes within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the antenna. As a result, the coil may be directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and is undesirable. To protect the coils, shields can be made from nonconducting materials, such as ceramics. However, many deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, it will soon become conducting, thus again substantially attenuating penetration of electromagnetic radiation into the plasma.